1. Field of the Invention
The present invention relates to an image forming apparatus configured to detect the size of a sheet to be used to form an image.
2. Description of the Related Art
In general, an image forming apparatus such as a copy machine and a printer uses a wide variety of sheet sizes. Examples of sheet sizes include A series sizes (A3, A4, A5, etc.), B series sizes (B4, B5, etc.), inch series sizes (legal (LGL), letter (LTR), etc.). Sheets of various sizes are used depending on a user's operating environment. Thus, a sheet size detection mechanism is provided in a sheet stacking portion. For example, Japanese Patent Application Laid-Open No. 7-112844 discusses a configuration for automatically detecting the size of a sheet by detecting the position of a regulation plate, which is configured to regulate the position of the sheet, when the sheet is set in a sheet stacking portion.
In a case where a sheet size detection mechanism is provided in a sheet stacking portion, the following problem arises. Specifically, the sheet size may not be detected accurately due to a user's operational error (e.g., omission of the adjustment of the position of the regulation plate at the time of setting the sheet). If continuous printing is performed in a state where the sheet size is not accurately detected, continuous printing that is not intended by the user may be performed. For example, an image may not be formed in an appropriate position on the sheet. This wastes sheets and toner. To prevent such wasteful printing, the following configuration is discussed. Japanese Patent Application Laid-Open No. 9-249338 discusses a configuration in which while the first sheet is fed from a sheet stacking portion, the length of the sheet is measured, and the measured sheet size is compared with a sheet size detected based on the position of a regulation plate. If the sheet sizes do not match, the continuous printing is stopped.
However, the following problem arises in a conventional image forming apparatus, although the continuous printing is stopped in a case where the sheet size detected based on the position of the regulation plate of the sheet stacking portion in the image forming apparatus does not match the sheet size measured while the sheet is fed. Specifically, since the fed first sheet has already been conveyed on a conveyance path of the image forming apparatus, there may be a case where an image forming operation on a photosensitive member has already started, resulting in wasteful consumption of sheets and toner. Wasteful consumption of sheets and toner is more likely to occur especially in the following case. For example, a difference in sheet length between A4 size (210 mm) and LTR size (216 mm) is 6 mm. To recognize the difference in sheet length based on the position of the regulation plate of the sheet stacking portion, the regulation plate needs to be set accurately. Accordingly, when an A4-size sheet is set, if the sliding amount of the regulation plate is insufficient by 6 mm, that is to say, if the regulation plate is set in a position for LTR size, which is included in a different size series, the image forming apparatus detects that a LTR-size sheet is stacked. However, since the size of the sheet that is actually stacked is A4 size, the image forming apparatus detects that an A4-size sheet is fed. As a result, the sizes do not match, causing wasteful consumption of sheets and toner.
As the foregoing describes, the conventional image forming apparatus has the problem that in a case where the user erroneously sets the regulation plate in an incorrect position, sheets and toner are wastefully consumed.